1. Field of the Invention
This invention relates to the production of aluminum-lithium alloys. More particularly, this invention relates to an improved process for continuous, in-line addition of molten lithium to a molten aluminum stream to form an aluminum-lithium alloy.
2. Description of the Prior Art
In the production of aluminum base alloys, it is common to add the alloying constituents as solids to molten aluminum in an open melting furnace. The alloying constituents, conventionally in the form of a master metal alloy or pure metals, are usually submerged beneath the surface of the molten aluminum to ensure faster melting with minimum oxidation of the alloying constituents. The molten mixture is then degassed to lower the hydrogen content of the melt by bubbling a gas, such as chlorine, argon and mixtures thereof, through the melt.
The production of aluminum-lithium alloys has become of increasing interest due to the combination of lightweight and high strength which such an alloy can be made to possess. However, the formation of aluminum-lithium alloys is significantly more difficult due to the reaction of aluminum-lithium alloys with refractory linings in the furnace, the rapid rate of oxidation of lithium and the concurrent generation of copious quantities of skim, hydrogen pickup by the molten alloy, objectionable fume evolution and composition gradients in the cast ingot due to the propensity of lithium to oxidize during processing of the molten alloy after the addition of lithium. In conventional processes, as much as 20 wt. % lithium added can be lost due to these undesirable mechanisms.
Attempts have been made to remedy these problems by, for example, adding the lithium to the melt after degassing of the molten aluminum. However, the need for uniformity of composition usually requires stirring which may promote oxidation as well as further hydrogen absorption.
It was, therefore, proposed in Balmuth U.S. Pat. No. 4,248,630 to use a special mixing crucible into which is poured molten aluminum, which has previously been degassed, and a separate stream of molten lithium. The two molten streams are blended together in the mixing crucible under a vacuum or inert atmosphere. After the correct quantities or ratios have been mixed, a valve is opened, and the aluminum-lithium mixture flows into an ingot casting mold.
However, there remains a need for a method of continuous in-line addition and mixing of molten lithium to a molten aluminum stream flowing into an ingot casting mold to ensure maximum uniformity in composition while minimizing oxidation losses, skim formation and hydrogen gas absorption by the molten mixture and lessening the requirements for using expensive refractories and reducing the replacement and maintenance of refractories by reducing the amount of refractory in contact with the molten aluminum-lithium alloy. The present invention resolves these problems and is capable of reducing the lithium loss to 3% or less, which is considered to be a marked advance in the art.